When X-ray radiation is projected through an examination subject, different absorption or scatter characteristics result as a function of the spectrum used and the substances that are to be penetrated, the proportion of low energies in the spectrum in relation to the high energies being absorbed more rapidly (beam hardening). With regard to the contrast or noise of the images that are to be reconstructed, it may therefore be advantageous to adapt the spectrum used to fit the planned projection through the examination subject. Basically, two approaches come into consideration for this purpose: On the one hand, the emitted spectrum may be regulated by way of the source, while on the other hand the detector may be embodied and if necessary be amenable to regulation, for example by means of filters, in such a way that it detects only defined ranges of a relatively wide incoming spectrum. An indicator for the spectral separation in this case is the spectral sensitivity distribution, which is given by the product yielded from the spectrum and the sensitivity of the detector.
A number of approaches for how the spectrum may be modified on the source side are known from real-world practice. These multi-energy techniques, as they are known, are categorized into the variants briefly described below. With so-called “kV switching”, the tube voltage, also known as the acceleration voltage, is varied at short time intervals over one or more readout cycles such that the electrons absorb different energies, finally resulting as bremsstrahlung (“braking radiation”) in different X-ray spectra. The “dual spiral” variant involves two scans of the same subject region which are performed using different X-ray spectra. The “split filter” approach represents a further variant, the prefilter of which consists of two different materials which allow different X-ray spectra to pass through. The X-ray radiation is then detected in each case only in the detector elements that are associated with the corresponding prefilter. Also known from real-world practice is the so-called “dual source” technique, in which two tube-detector systems generate the X-ray radiation simultaneously with different X-ray spectra and measure these separately from one another.
Previously, the projection data was acquired in most cases using detectors which integrate across the entire energy range, referred to as the total spectrum, of the X-ray radiation in order to acquire measurement data. In order to achieve a certain energy resolution, the detectors may also be arranged in two layers as a “dual layer detector”, with predominantly the low-energy quanta being detected in the first layer and the remaining higher-energy X-ray quanta being detected in the second layer.
In contrast thereto, photon-counting detectors measure the input spectrum in a spectrally resolved manner. In this case, depending on the number of thresholds implemented, a plurality of spectrally different datasets are generated. Because there is only one input spectrum, however, the spectral separation of the individual datasets may in this case be less good than in the case of the previously described multi-energy approaches. The use of such a photon-counting detector in an imaging system is described in the publication DE 102007027460 A1.